In recent years, the automobile industry is growing remarkably and the desired values of performances of individual automotive parts separately constituted including greases increase steadily year by year. To provide a new technique is always required.
Especially in electric power steering apparatus for motor vehicles, technical innovations are remarkable. The apparatus, which at first were employed in solar cars and part of light cars, have recently come to be mounted in an exceedingly wide range from small passenger cars to large passenger cars. In this field, the number of cars equipped with the apparatus doubles year by year.
Electric power steering apparatus for motor vehicles are explained below in detail. The automotive power steering apparatus mainly used at present are of the hydraulic type. In the case of the hydraulic type, however, it is necessary to directly take a measure not to cause an environmental problem because a hydraulic fluid (power steering fluid) is used. In addition, since the hydraulic pump for generating a hydraulic pressure is operated by a power from the engine and is always operated (even when the steering wheel is not manipulated), use of the hydraulic apparatus is accompanied by an engine power loss. This is a cause of impairment of fuel efficiency.
On the other hand, in the case of an electric power steering apparatus, an electric motor is used as a power source for power assist. It is sufficient to drive the electric motor with a controller only when power assist is necessary. Furthermore, since the electric motor is driven by the electricity generated during car running, the engine power loss is extremely small and the effect of saving fuel cost is high. Consequently, the amount of energy consumption also can be far smaller than in the hydraulic type.
However, the electric power steering apparatus presently in use are still inferior in output power to hydraulic power steering apparatus. It is hence important to improve the ability of the electric motor and to minimize the friction of individual constituent parts and thereby minimize the burden to be imposed on the motor.
Furthermore, in electric power steering apparatus, low-temperature starting is regarded as important especially in cold districts. The reasons for this are as follows. In the case of hydraulic power steering apparatus, engine warming-up has an effect that the heat source of the hydraulic pump, which is directly connected to the engine, warms up each part of the steering apparatus using the hydraulic oil as a heat medium. Because of this, a lubricant having general low-temperature characteristics has sufficed for the hydraulic steering apparatus. In the case of electric power steering apparatus, however, the steering apparatus is not readily warmed up because of the absence of a direct heat source from the engine.
Consequently, it is essential that the greases to be used in parts for the apparatus should be ones which attain stable friction torque properties at low temperatures.
Motor vehicles are used all over the world. Electric power steering apparatus also are hence designed and produced while taking account of common use in environments ranging from an extremely cold environment of about −40° C. to a red-heat environment of 100° C. or higher [(outside air temperature)+(radiant heat in engine room)+(radiant heat from road surface)]. There is a desire for a grease which has stable low-torque characteristics in a wide temperature range from a low temperature to a high temperature, does not suffer the oil film deficiency caused by a viscosity decrease at high temperatures, and has a long life corresponding to the life of the vehicle.
Furthermore, there recently are rapidly increasing trends toward the active incorporation of subtle and delicate behaviors according to natural laws, such as the vibrations, fluctuations, and fuzzy movements which are sensed by men, into automotive parts and toward an offer of more comfortable and stable handling and a more comfortable interior space. New techniques have rapidly come to be desired.
Electric power steering apparatus are steering apparatus which will be mainly used in the future in place of the hydraulic steering apparatus and, hence, are progressing in an innovational manner. Under these circumstances, the introduction of unconventional new ideas and techniques is remarkable. One of important subjects is to evolve subtle feelings sensed by men, such as those described above, in a more comfortable direction. This tendency is exceedingly strong especially in high-class passenger cars.
A specific example of such feelings related to the invention is as follows. In the case where a car is running straight on a straight highway, the steering apparatus is in a load-free state and, hence, the electric motor which operates steering is in the state of being stopped and giving no assist. In this situation, the driver's hands gripping the steering wheel are not in a completely stationary state. The tires are moving little by little because of road surface undulations, etc. and the steering wheel, which receives such subtle vibrations and movements, subtly swings in the leftward/rightward directions. The driver's hands gripping the steering wheel sense these subtle movements, and the driver is continuously in the state of driving the car while manipulating the steering wheel little by little in the leftward/rightward directions.
In this period of subtle manipulation, electric assist is not given. Consequently, the subtle feelings given to the driver's hands are attributable to the direct transmission of frictions generated at individual parts constituting the steering system. (In the case where the steering wheel is manipulated and electric assist is given, the frictions generated at the individual parts are countervailed by the power of electric assist and are hence difficult to be transmitted to the driver's hands.)
Examples of the frictions thus occurring at individual parts constituting the steering mechanism include frictions at the following parts which are shown in the FIG. 2 given in patent document 1: the sliding parts of the sealed bearings 33 and 34; the contact part between the teeth of the rack shaft 15 and the teeth of the pinion shaft 22; and the sliding part between either the ball screw groove 15b or the ball screw groove 38A and the ball bearing 39 interposed between these grooves.
When such frictions at the sliding parts of individual parts fluctuate irregularly, there is a high possibility that these fluctuations are directly transmitted to the driver's hands grasping the steering wheel and are sensed as an unpleasant feeling. Although the irregular fluctuations of friction occurring here practically exert no influence on steering wheel manipulation, the subtle feeling given to men is a serious problem in cars of the kind for which exceedingly high quality and ultimate comfortableness are pursued, such as high-class passenger cars.
In the case of ball screw devices having the same mechanism as the ball screw structure in the electric power steering apparatus described above, the ball screw devices are generally employed mainly in machine tools. However, the mechanism of the rolling/sliding of balls is almost the same and use of general lubricating greases is apt to result in friction fluctuations at the sliding part between the balls and a screw part. In case where irregular friction fluctuations have occurred, processing accuracy decreases and higher quality cannot be imparted to the work.
There are many documents on rolling devices, electric power steering apparatus, and lubricants for the apparatus. However, there is no document on a rolling device which discloses an electric power steering apparatus in which the technique concerning a grease composition according to that patent document is used to diminish the subtle unpleasant feeling given to men and thereby enable more comfortable manipulation and which further discloses a technique for inhibiting the irregular friction fluctuations.
For example, documents on rolling devices concerning lubrication include patent documents 2 to 5.
Patent document 2 discloses a technique for prolonging the lubrication life of a high-load ball screw to be incorporated into the injection drive shaft or mold clamping mechanism driving part of an injection molding machine of the electrically operated type or into an electric press, bender, or the like. In this technique, a grease containing a urea compound, an organonickel compound, and an organomolybdenum compound and having a 100,000-time worked consistency as measured in accordance with JIS K-2220 of 280 or higher is enclosed to thereby inhibit the balls of the ball screw from being marred and thereby attain life prolongation. Although the urea compound shown in this patent document and part of the additives shown therein closely resemble part of the grease composition of the present invention, the grease composition disclosed in that document basically differs from the grease composition of the invention and utterly differs in the effect obtained.
Patent document 3 discloses a grease for the ball screw in the driving apparatus of an electrically operated injection molding machine. This grease comprises a urea compound as a thickener and a mineral oil or synthetic oil having a viscosity at 40° C. of 300 mm2/s or higher as a base oil, and is characterized by having a consistency of 300 or higher. The technique disclosed is intended to prevent early damage. Although part of the urea compound thickener shown in the patent document closely resembles part of the grease composition of the invention, the grease composition disclosed therein basically differs from the grease composition of the invention and utterly differs in the effect obtained.
Patent document 4 discloses a technique concerning a lubricating grease for lubricating the lubrication passageways of a ball screw in which a screw groove has been formed in each of the outer peripheral surface of the screw shaft and the inner peripheral surface of the nut. This lubricating grease has a lightness of 1 or higher and has a composition obtained by incorporating additives including an organomolybdenum compound into a lubricating grease prepared by thickening a base oil having a viscosity at 100° C. of 3.0-7.5 mm2/s with a urea compound thickener. The effects of this technique are as follows. Since the lubricating grease enclosed has a lightness of 1 or higher, the lubricating grease changes to black upon deterioration and decreases in lightness to below 1. Consequently, the timing of greasing can be easily noticed. Furthermore, the use of a urea compound thickener in combination with a base oil having a viscosity at 100° C. of 3.0-7.5 mm2/s enables the lubricating grease to be smoothly supplied to the friction surfaces to thereby improve the durability of the ball screw. Although part of the urea compound thickener and part of the additives in the grease composition disclosed in the patent document closely resemble part of the grease composition of the invention, the grease composition disclosed therein basically differs from the grease composition of the invention and utterly differs in the effect obtained.
Patent document 5 discloses a technique concerning a method of lubricating a ball screw with a lubricating oil which comprises a poly(α-olefin) oil, an alkylbenzene oil, and an ethylene/propylene oligomer as a base oil and contains an extreme-pressure agent such as an ash-free thiocarbamate compound, hydroxymolybdenum dithiocarbamate sulfide, or the like and which has a dynamic viscosity at 40° C. of 10-460 mm2/s. This technique is for preventing the abnormal wearing and seizing of a ball screw by diminishing frictional heat in the sliding part or removing wearing dust and foreign matters. The lubricant disclosed in the patent document is a lubricating oil and intrinsically differs from the grease composition of the invention. This lubricating oil utterly differs in the effect thereof to the technique of the invention.
There also are patent documents 6 to 11 as documents on an electric power steering apparatus and a lubricant for the apparatus.
Patent document 6 describes a grease composition for automotive steering mechanisms which comprises (a) thickener, (b) a base oil having a pour point of −40° C. or lower, (c) an organomolybdenum compound, (d) melamine cyanurate, (e) polytetrafluoroethylene, and (f) molybdenum disulfide. There is a statement therein to the effect that the grease composition for automotive steering mechanisms exhibits adequate lubricity in a gear toothing part especially in a rack-and-pinion part, the hypoid gears of a pinion-assisted electric power steering mechanism, or the like. However, the makeup of this grease composition utterly differs from that of the grease composition of the invention.
Patent document 7 describes an electric power steering apparatus employing as a lubricant a grease which has improved long-lasting high-temperature lubricity while retaining intact low-starting-torque characteristics at low temperatures. There is a statement therein to the effect that the grease comprises a base oil which is a synthetic hydrocarbon oil, a thickener selected from lithium composite soaps and urea compounds, and a lubricity improver selected from solid lubricants and oily substances. There also is a statement therein to the effect that the electric power steering apparatus is a power steering apparatus which has an electric motor for generating steering assist power and a speed reducer serving to reduce a rotation speed by means of a gear mechanism connected to the rotation shaft of the motor and in which at least one of the reduction gears of the gear mechanism is made of a synthetic resin and this synthetic-resin gear is lubricated with a grease. With respect to the grease composition, a claim includes the term urea compound thickener. This grease composition is coincident in this point only. However, details of the makeup of the composition and an effect are not shown at all.
Patent document 8 describes a lubricating grease composition which comprises a base oil and a thickener and in which a fluororesin powder has been added to the base oil. There is a statement therein to the effect that this grease composition is used in e.g., the speed reducer of an electric power steering apparatus.
Patent document 9 describes a lubricating grease composition which comprises a base oil and a thickener and in which the thickener comprises a combination of lithium stearate and lithium hydroxystearate. There is a statement therein to the effect that this grease composition is used in, e.g., the speed reducer of an electric power steering apparatus.
That speed reducer of an electric power steering apparatus which is described in patent document 8 and patent document 9 and the specific part therein to be lubricated are, for example, a speed reducer employing a worm wheel made of a synthetic resin, such as a polyamide resin. There is a statement therein to the effect that the role of a lubricating grease composition contributing to friction reduction in the part to be lubricated in the sliding part (friction surfaces) between the worm wheel made of a synthetic resin and the worm shaft made of a metal is important. However, this grease composition utterly differs from the grease composition of the invention.
Patent document 10 describes a grease composition for resin lubrication obtained by incorporating montan wax into a grease comprising a thickener and a base oil. There is a statement therein to the effect that the grease composition is for the speed reducer of an electric power steering apparatus and the specific part to be lubricated therewith is the speed reduction mechanism part including a worm wheel gear made of a resin (polyamide) and a worm gear made of steel. With respect to the grease composition, a claim includes the term urea compound thickener. This grease composition is coincident in this point only. However, this grease composition utterly differs from the grease composition of the invention as apparent from the facts that it contains montan wax as an essential ingredient and that it is a grease composition for resin lubrication.
Patent document 11 describes a lubricating grease composition which comprises a base oil and a thickener and in which a polyethylene oxide wax has been incorporated. There is a statement therein to the effect that the grease composition is used for, e.g., the speed reducer of an electric power steering apparatus. The speed reducer of an electric power steering apparatus is a speed reducer comprising a worm wheel made of a synthetic resin and a worm shaft made of a metal, and the specific part to be lubricated with the grease composition is the friction surfaces of this speed reducer. However, this grease composition utterly differs from the grease composition of the invention.
Patent document 12 discloses an electric power steering apparatus in which the assist power of an electric motor is transmitted to the steering mechanism of a motor vehicle through a speed reduction gear mechanism. This electric power steering apparatus is characterized in that the follower gear of the speed reduction gear mechanism comprises a metallic core pipe and, integrally formed on the outer periphery of the pipe, a resinous part made of a resin composition and having gear teeth formed in the periphery thereof and that the speed reduction gear mechanism is lubricated with a grease composition which contains a wax having a polar group introduced in the molecular structure thereof and employs a thickener such as a diurea compound. In this apparatus, the sliding lubrication of the interface between the resinous member and a metallic member, which is the sliding part of the speed reduction gear mechanism, is satisfactorily maintained over long to thereby give an excellent steering feeling. Although part of the diurea compound and part of the additive ingredients in the grease composition disclosed in the patent document closely resemble part of the grease composition of the invention, the grease composition disclosed therein basically differs from the grease composition of the invention. In addition, the electric power steering apparatus utterly differs in constituent parts and friction mechanism.
The electric power steering apparatus according to the invention is an apparatus shown in the accompanying drawings in patent document 1, i.e., a rack-assisted electric power steering apparatus constituted of a ball screw mechanism 37 and rolling bearings 33 and 34. This apparatus gives power assist in the shaft directions based on the ball screw connected to the rack shaft. Because this ball screw mechanism closely resembles the ball screw mechanisms mounted in machine tools and the like, the lithium grease frequently employed in these machine tools has been used as the lubricating grease for that part.
The rack-assisted electric power steering apparatus disclosed in patent document 1 is one in which the electric motor has been disposed coaxially with the rack shaft. However, there also are the type in which the electric motor has been disposed not coaxially but parallel with the rack shaft (e.g., patent document 13) and the type in which the electric motor has been disposed so that the shaft axis thereof intersects that of the rack shaft (e.g., patent document 14). In these types, the electric motor is connected to the ball screw mechanism (ball screw nut) through a transmission device such as a gear mechanism or a belt.
However, as the electric power steering apparatus improve in assist power, durability, and stable low-torque characteristics and remarkably progress in other various performances with the increasing desire for the mounting of the apparatus in motor vehicles, the lubricating greases according to prior-art techniques have become unable to enable the steering apparatus to sufficiently exhibit stable steering characteristics at temperatures ranging from a high to a low temperature and have a satisfactory durability life. In addition, the prior-art lubricating greases have had a problem concerning low-torque characteristics in lower-temperature environments.
The “rolling bearing” employed in each of the electric power steering apparatus described above mainly is a deep-groove ball bearing having one or more grooves (see, for example, patent document 15).
The “electric power steering apparatus” according to the invention should not be construed as being limited to those described above, and can be one such as those described in, e.g., patent document 13 and patent document 14. In the former case, the ball screw mechanism 2 and each ball bearing described in the document correspond to the “rolling bearing” and, in particular, the bearings 8 and 10 which movably support the nut part 2a correspond to the “ball bearing which utilizes the rolling of balls”. In the latter case, the ball screw mechanism 9 (19, 29) and each ball bearing described in the document correspond to the “rolling bearing” and, in particular, the bearing 10 (20) which movably supports the nut part 9a (19a, 29a) corresponds to the “ball bearing which utilizes the rolling of balls”.
The rolling device is a rolling device utilizing the rolling of balls, and rolling devices having any ball screw mechanism are included therein.
The present inventors made thorough investigations on constituent parts of the electric power steering apparatus described especially in patent document 1 as to which parts are apt to give a subtle unpleasant feeling to men. As a result, they found that constituent parts suffering rolling/sliding friction, such as the sealed bearings 33 and 34 and the sliding part between either the ball screw groove 15b or the ball screw groove 38A and the ball bearing 39 interposed between these grooves, are apt to undergo irregular friction fluctuations. It was further found that the degree of these irregular friction fluctuations is considerably influenced by the difference of the lubricating grease. The inventors made further intensive investigations. As a result, they have found that the lubricating grease composition of the invention can greatly reduce the irregular friction fluctuations occurring at those constituent parts and is an extremely stable lubricating grease composition which does not give an unpleasant feeling to the driver at all. Based on these findings, the inventors have succeeded in providing a rolling device and an electric power steering apparatus each containing the lubricating composition.
Patent Document 1: JP-A-2003-335249
Patent Document 2: JP-A-2000-303089
Patent Document 3: JP-A-2001-49274
Patent Document 4: JP-A-2001-304307
Patent Document 5: JP-A-2002-340132
Patent Document 6: JP-A-2001-64665
Patent Document 7: JP-A-2002-308125
Patent Document 8: JP-A-2002-363589
Patent Document 9: JP-A-2002-363590
Patent Document 10: JP-A-2002-371290
Patent Document 11: JP-A-2003-3185
Patent Document 12: JP-A-2004-301268
Patent Document 13: JP-A-2004-114972
Patent Document 14: JP-A-2004-122858
Patent Document 15: JP-A-2004-144118